Magnet system for producing a magnetic field



Se t. 3, 1957 w. F. K. J. NIKLAS EI'AL 2,805,351

MAGNET SYSTEM FOR PRODUCING A MAGNETIC FIELD Filed Sept. 4, 1953 INVENTORS WILFRID FRANZ KONRAD JOHANN NIKLAS JAN VAN WUNGAARDEN.

ALPHQNSUS psmus \2 ROOY AGENT United States Patent MAGNET SYSTEM FOR PRODUCING A MAGNETIC FIELD Application September 4, 1953, Serial No. 378,602

Claims priority, application Netherlands September 10, 1952 7 Claims. ((31. 313--76) Jan van Wijn- This invention relates to magnet systems for producing a magnetic field in a space traversed by a beam of electrically charged particles, the systems comprising a magnet and two pole pieces of magnetically soft material situated one at each side of the beam.

The control of a beam of electrically charged particles by means of a magnetic field is a problem involved in various fields of electrical engineering. By way of example reference is made to cathode-ray tubes, for instance for oscillography or television purposes, mass spectrographs and amplifying tubes. For the sake of simplicity a cathode-ray tube has hereinafter been chosen as an example without, however, the invention being limited to this type of tubes.

In cathode-ray tubes the position of the electron beam is often to be influenced magnetically in their cross-sectional area. This is necessary, for example, in so-called ion-traps and very desirable in tubes in which the electron beam is electrostatically focussed. Many diflferent constructions of magnet system comprising a magnet having connected to it two pole shoes situated one at each side of the space traversed by the electron beam are known for this purpose. Thus, for example, a magnet system has been described, which comprises a small permanent magnet situated adjacent the neck of the cathoderay tube, the pole shoes secured to the magnet consisting of a narrow strip of soft iron provided with two ends. Seen from the magnet, said pole shoes first extend parallel, then diverge and finally extend parallel again, the neck of the cathode-ray tube being located between the two last-mentioned parallel ends.

The magnetic field at the electron beam is naturally determined by the magnetic lines of force issuing from the magnet and the pole shoes. It has been found that the magnetic configuration of the field in known constructions involves very troublesome astigmatism of the beam. The icon astigmatism is to be understood to mean the phenomenon that the cross-sectional area of an initially cylindrical beam in a plane at right angles to the main direction of the particles of the beam is ellipsoidal. Researches on which the invention is base have revealed that the main cause of said astigmatism is that the magnetic field strength, measured on a line through the beam midway between the pole shoes, decreases materially accordingly as the distance from the magnet increases. One reason is that the pole shoes have a certain reluctance.

The present invention provides a magnet system permitting the astigmatism of the beam to be materially reduced.

A magnet system according to the invention for producing a magnetic field in a space traversed by a beam of electrically charged particles comprises a magnet having connected to it two pole shoes of magnetically soft material situated one at each side of the beam and extending substantially in a plane at right angles to the beam, the system having the feature that the reluctance of the pole shoes decreases from one half of the length of the poleshoes onwards, towards their free' ends seen from the magnet.

The decrease in reluctance is obtainable in several different manners. If the pole shoes consist of strip-shaped material, that is to say strips of uniform thickness throughout their length, whose broad sides face the beam, the width of the pole shoes must increase from one half of their length onwards. Alternatively, the width of the pole shoes is uniform throughout their length but their thickness increases. In a third embodiment of the invention, the permeability per unit volume increases from one half of the length of the pole shoes onwards. The lastmentioned possibility can readily be realized when using ferrites, for example.

In order to obtain a very simple construction, the reluctance may be caused to diminish from the magnet onwards, instead of diminishing from one half of the length of the pole shoes onwards. This yields in many cases an additional decrease in astigmatism.

The magnet system is preferably symmetrical relatively to a plane through the beam and the centre of gravity of the magnet.

Preferably, a permanent magnet is used. Alternatively, however, an electro-magnet may be used.

In order that the invention may be readily carried into effect it will now be described with reference to the accompanying drawing, given by way of example, in which Fig. 1 represents diagrammatically a magnet frame system according to the invention, in which the pole shoes consist of strip-shaped material,

Fig. 2 is a side view of the magnet system shown in Fig. 1,

Fig. 3 is a graphical representation of the field-strength distribution obtained by means of a magnet frame shown in Figs. 1 and 2, and

Fig. 4 shows diagrammatically a magnet system according to the invention, in which the pole shoes are of uniform width throughout their length.

With reference to Fig. l, the reference numeral 1 denotes the cross-section of the neck of a cathode-ray tube producing an electron beam 2. Laterally of the neck 1 are provided two strip-shaped pole shoes 3, 4 connected at 5 and 6 respectively to the permanent magnet 7 and, as may be seen from Fig. 2, increasing in width from half their length onwards. The magnetic field strength on the line 3-8 passing through the electron beam 2 and the centre of gravity of the magnet 7 may be seen from the graph shown in Fig. 3, wherein the field strength in Gauss, obtained by means of a given magnet 7, is plotted on the abscissa, the distances on the line 8-8 being plotted on the ordinate. The field strength on said line varies in accordance with the curve 9. Should the pole shoes 3, 4 have the same width throughout, as indicated in broken lines in Fig. 2, the field strength would vary in accordance with curve 10. In the construction shown in Fig. l, the curve 9, particularly at the height of the electron beam, is much flatter than the curve 10, owing to which a lesser degree of astigmatism occurs.

Fig. 4 shows a magnet system yielding approximately the same distribution of the field strength as the system shown in Fig. 1. In this instance, the pole shoes 11 and 12 are of uniform width but their thickness increases from the magnet 13 onwards.

What is claimed is:

l. A magnet system comprising a pair of substantially parallel spaced magnetically soft pole shoes, and means interconnecting one pair of opposed ends of said shoes for producing a magnetic field between said shoes, each of said shoes having a portion at about its center with a predetermined reluctance and a portion at the end thereof remote from the producing means of lesser reluctance,

the intervening portions between said center and end portions having reluctances varying continuously therebetween.

2. A magnet systcmcomprising a pair of approximately parallel spacedstrip-shaped soft ferromagnetic members, and a permanent magnet interconnecting one pair of opposed ends of said members, the reluctance of each of said members varying continuously and uniformly from a lower value to a higher value between the end thereof remote from the magnet and a portion thereof lying close to the magnet.

3. A magnet system as set forth in claim 2 wherein the members each have a uniform thickness and an increasing width from the magnet to the end remote therefrom.

4. A magnet system as set forth in claim 2 wherein the members each have a um'form width and an increasing thickness from the magnet to the end remote therefrom.

5. A magnet system as set forth in claim 2 wherein the members each consist of a material whose permeability per unit volume of the member gradually increases from the magnet to the end remote therefrom.

6. A magnet system as set forth in claim 5 wherein the material consists of fired ferrites.

7. In combination; an envelope, means Within said envelope for producing a beam of electrically charged particles in a given path, and a magnet system surrounding said envelope for producing a magnetic field therein, said system including a pair of magnetically soft pole shoes each disposed on one side of said beam and extending substantially in a plane at right angles tothe beam path, and a magnet interconnecting one pair of ends of said pole shoes, the center of said pole shoes being approximately at the center of said beam, the reluctance of each of the pole shoes gradually decreasing from at least about its center to the end thereof remote from the magnet.

References Cited in the file of this patent UNITED STATES PATENTS 2,264,567 Gunther Dec. 2, 1941 2,539,156 Ostreicher Ian. 23, 1951 2,552,342 Mucher May 8, 1951 

